Vim9: no proper error for trying to declare optional argument in lambda

[lacygoill]

Describe the bug

In Vim9 script, a confusing error is given when we try to declare an optional argument in a lambda.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    var Ref: func(any, ?any): bool
    Ref = (_, v = 0) => true
EOF
)

E1181 is raised:

E1181: Cannot use an underscore here

Expected behavior

A different error is raised, like this one:

E1234: cannot use "name = value" in lambda

Or this one:

E1234: invalid argument name for lambda

Environment

• Vim version: 8.2 Included patches: 1-3048
• OS: Ubuntu 20.04.2 LTS
• Terminal: XTerm(353)

Additional context

The current error message is misleading. We can use an underscore in a lambda:

vim9script
var Ref = (_) => 'no error'
echo Ref('ignore me')

no error

And in a regular function, we can use an optional argument after _:

vim9script
var Ref: func(any, ?any): any
def Func(_, v = 'no error'): any
    return v
enddef
Ref = Func
echo Ref('ignore me')

no error

The issue is simply that the optional_argument = value syntax is not supported in a lambda.

---

Same issue when we wrongly use the ?type syntax (which is only valid when specifying the type of a funcref; not the type of a lambda's argument):

vim9script
var Ref: func(any, ?any): bool
Ref = (_, ?v) => true

E1181: Cannot use an underscore here